1. Field of the Invention
The present invention relates to an oxygen-absorbing multi-layer laminate, a production method thereof and a packaging container. More specifically, the present invention relates to an oxygen-absorbing multi-layer laminate suitable for food preservation, etc. due to its excellent ability of preserving flavor and taste of food, a method of efficiently producing the oxygen-absorbing multi-layer laminate and a packaging container made of the oxygen-absorbing multi-layer laminate.
2. Description of the Prior Art
A recent development in the technique of oxygen-absorbing packaging has provided a packaging container made of a multi-layer laminate having an oxygen-absorbing layer made of an oxygen-absorbing resin composition comprising a thermoplastic resin and an oxygen-absorbing agent. With such a construction, the packaging container has been improved in gas barrier properties and imparted with an oxygen-absorbing function. An oxygen-absorbing multi-layer laminate comprising an intermediate oxygen-absorbing layer between a gas barrier outer layer and an oxygen-permeating inner layer is usually fabricated into an oxygen-absorbing packaging container with multi-layer structure, for example, a bag, cup, tray, bottle, etc., due to their easiness of forming.
Japanese Patent Application Laid-Open Nos. 2-72851 and 4-90848 disclose oxygen-absorbing multi-layer laminates and films having an oxygen-absorbing layer made of a resin dispersed with an iron-based oxygen-absorbing agent. Japanese Patent Application Laid-Open No.8-72941 proposes a technique for improving the oxygen-absorbing ability of an oxygen-absorbing multi-layer laminate. Furthermore, Japanese Patent Application Laid-Open Nos. 8-132573 and 9-40024 disclose oxygen-absorbing multi-layer laminates in which a polyolefin layer is interposed between a resin layer containing an oxygen-absorbing agent and a gas barrier layer.
Generally, in the production of the oxygen-absorbing multi-layer film or sheet, a compound prepared by kneading an iron-based oxygen-absorbing agent into a thermoplastic resin such as polyolefin is melted and laminated on another resin layer to form an oxygen-absorbing layer. In this method, since the preparation of the compound and the lamination process thereof inevitably include a step of melting at a high temperature for a certain period of time, an offensive odor is generated due to degradation of the molten resin by the action of the oxygen-absorbing agent present therein, and the resulting film or sheet has a poor appearance due to the vaporization of the moisture in the oxygen-absorbing composition. In addition, to facilitate the lamination process, the oxygen-absorbing layer is required to have a suitable level of thickness, this likely affecting the bending properties of the resulting oxygen-absorbing multi-layer film.
The oxygen-absorbing ability of an oxygen-absorbing multi-layer laminate or a packaging container made thereof is evaluated by an oxygen-absorbing rate per unit area of the oxygen-absorbing multi-layer laminate as well as by an oxygen-absorbing capacity defined as a maximum amount of oxygen being absorbed per unit area of the oxygen-absorbing multi-layer laminate.
The oxygen-absorbing rate depends mainly on a content of the oxygen-absorbing agent in the oxygen-absorbing multi-layer laminate and a permeating speed of oxygen from the surface of a packaging container until reaching the oxygen-absorbing agent in the oxygen-absorbing layer. The oxygen-absorbing capacity depends largely on the content of the oxygen-absorbing agent in the oxygen-absorbing multi-layer laminate.
To obtain an oxygen-absorbing multi-layer laminate having a high oxygen-absorbing ability, i.e., a large oxygen-absorbing rate and a large oxygen-absorbing capacity, the oxygen-absorbing resin layer in the multi-layer laminate is needed to be made of a resin compound containing the oxygen-absorbing agent in a high content. However, the flowability of the compound drastically decreases with increasing content of the oxygen-absorbing agent to make it difficult to prepare the compound and extrude it into a film with uniform thickness from a T-die extruder. For this reason, there is a practical limit in increasing the content of the oxygen-absorbing agent in the compound.
Alternatively, the oxygen-absorbing capacity may be improved by increasing the thickness of the oxygen-absorbing resin layer while controlling the oxygen-absorbing agent therein to a low content. However, the oxygen-absorbing rate is not improved sufficiently by merely increasing the thickness of the oxygen-absorbing resin layer, thereby failing to obtain a substantial improvement in the oxygen-absorbing ability. In addition, an increased thickness of the oxygen-absorbing resin layer, in turn, unnecessarily increases the total thickness of the oxygen-absorbing multi-layer laminate to result in several problems such as deterioration of the properties required for the packaging material, a poor secondary processability in thermoforming, bag making, etc. and an increased material cost.
Japanese Patent Application Laid-Open No. 55-116434 and Japanese Patent Application Late Laid-Open No. 6-506140 disclose a label oxygen absorber produced by fixing an iron-based oxygen-absorbing agent on an adhesive layer applied on a base film and then disposing a cover sheet such as a gas-permeable resin film, etc. so as to adhere the cover sheet to the base film through a portion of the adhesive with no oxygen-absorbing agent. However, there is a fear of leakage of the oxygen-absorbing powder separated from the adhesive layer due to insufficient fixing as well as a fear of inhibition of the oxygen-absorbing reaction by water penetrated into the space between the iron powder and the cover sheet. Therefore, the proposed label oxygen absorber is not suitable for preserving foods of high water content.
In addition, the oxygen-absorbing multi-layer laminate generally suffers from a problem of deterioration in appearance, film strength and heat sealability when the oxygen-absorbing agent in the oxygen-absorbing resin layer is exposed on the surface contacting foods.
Accordingly, a first object of the present invention is to provide a oxygen-absorbing multi-layer laminate capable of saving cost in practical use and excellent in the oxygen-absorbing ability, mechanical strength, heat sealing properties, etc. by avoiding the drawbacks encountered in the conventional oxygen-absorbing multi-layer laminate produced by fabricating a compound incorporated with the oxygen-absorbing agent, namely, drawbacks such as separation and leakage of the oxygen-absorbing particle due to insufficient fixing and generation of odor due to deterioration of resin through heat-melting.
A second object of the present invention is to provide a method for efficiently producing the oxygen-absorbing multi-layer laminate having a high cost-performance and excellent properties.
A third object of the present invention is to provide a packaging container made of the oxygen-absorbing multi-layer laminate having a high cost-performance and excellent properties, which is suitable for use in preserving foods due to its excellent oxygen-absorbing ability and long storage life for retaining the flavor and taste of foods.
As a result of an intense search in view of the above objects, the inventors have found that a laminate having two layers or more, in which particles of the oxygen-absorbing agent are locally distributed between adjacent layers or the adjacent layers are bonded to each other through interposed particles of the oxygen-absorbing agent to form an integral part, is suitable as the oxygen-absorbing multi-layer laminate meeting the above object. Also, the inventors have found that an oxygen-absorbing multi-layer laminate having a gas barrier layer disposed on one of the surfaces thereof, or having an oxygen-permeating layer on one of the surfaces thereof and a gas barrier layer on the other surface thereof is particularly suitable for the first object.
In addition, the inventors have found that the second object can be achieved by uniformly sprinkling particles of the oxygen-absorbing agent on the surface of a first thermoplastic resin layer and then thermally press-bonding a second thermoplastic resin layer onto the sprinkled surface.
The inventors have further found that a packaging container, at least a part of the surface thereof being made of the above oxygen-absorbing multi-layer laminate having at least a gas barrier layer laminated on one of the surfaces thereof, can meet the third object.
The present invention has been accomplished based on the above findings.
Thus, the present invention provides:
(1) An oxygen-absorbing multi-layer laminate comprising a first thermoplastic resin layer, a second thermoplastic resin layer and an oxygen-absorbing agent in which particles of the oxygen-absorbing agent are locally interspersed between the first and second thermoplastic resin layers (first oxygen-absorbing multi-layer laminate);
(2) An oxygen-absorbing multi-layer laminate comprising a first thermoplastic resin layer, a second thermoplastic resin layer and an oxygen-absorbing agent in which the first and second thermoplastic resin layers are integrally bonded to each other while interposing particles of the oxygen-absorbing agent (second oxygen-absorbing multi-layer laminate);
(3) An oxygen-absorbing multi-layer laminate having at least three thermoplastic resin layers in which particles of the oxygen-absorbing agent are locally interspersed between adjacent layers of at least one adjacent pair (third oxygen-absorbing multi-layer laminate);
(4) An oxygen-absorbing multi-layer laminate having at least three thermoplastic resin layers in which each pair of adjacent layers are integrally bonded to each other and particles of the oxygen-absorbing agent are interposed between integrally bonded adjacent layers of at least one adjacent pair (fourth oxygen-absorbing multi-layer laminate);
(5) Oxygen-absorbing multi-layer laminates (1) to (4), each further having a gas barrier layer on at least one of the surfaces;
(6) Oxygen-absorbing multi-layer laminates (1) to (4), each further having an oxygen-permeating layer on at least one of the surfaces and a gas barrier layer on the opposite surface;
(7) A method of producing an oxygen-absorbing multi-layer laminate, comprising a step of uniformly sprinkling particles of an oxygen-absorbing agent on the surface of a first thermoplastic resin film, and a step of thermally press-bonding a second thermoplastic resin film onto the sprinkled surface of the first thermoplastic resin film; and
(8) A packaging container in which at least a part of the surface thereof is made of the oxygen-absorbing multi-layer laminate (5) or (6).